1. Field of the Invention
The present invention relates to a magnetic recording medium for use in digital magnetic recording, a production method thereof, and a magnetic disc apparatus. More specifically, the present invention relates to a magnetic recording medium which can increase the track density and enhance thermal stability in a discrete track medium or can increase the bit density and enhance thermal stability in a bit patterned medium, and a production method thereof. The present invention also relates to a magnetic disc apparatus having mounted therein this magnetic recording medium.
2. Description of the Related Art
In the magnetic recording field, in order to cope with high-density recording of a medium of a magnetic disc apparatus, meet the requirement for larger capacity than in conventional mediums and solve the problem of thermal fluctuation, a so-called patterned medium (also called “patterned media”) is being applied. The patterned medium includes a discrete track medium (DTM) where recording tracks are separated with the intention of bringing about interference between tracks, and a bit patterned medium (BPM) where recording bits are separated with the intention of enhancing thermal stability.
Conventionally, in the production of a discrete track medium, a method of etching a magnetic recording layer to form a track (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2007-257801) or a method of performing ion implantation in a magnetic recording layer to weaken the magnetism in that portion, thereby forming a track (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2007-273067) is used. Also, in the production of a bit patterned medium, a method of etching a magnetic recording layer to separate a magnetic recording region corresponding to single-bit recording (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2007-250091) is used. The production methods of a medium described in these patent publications are specifically described below.
FIGS. 1A to 1F are cross-sectional views sequentially showing the method for producing a discrete track medium by etching described in JPP'801. First, as shown in FIG. 1A, a medium obtained by sequentially stacking a ferromagnetic recording layer 152, a protective layer 153 and a resist layer 154 on a glass substrate 151, and a stamper 171 having troughs and ridges in a reversal pattern of the desired magnetic pattern are prepared. Then, although not shown, the troughs and ridges of the stamper are pressed against the resist layer to transfer the trough-ridge pattern and through a series of steps such as etching and ion milling, a medium shown in FIG. 1B is produced. In the obtained medium, as shown, the portion corresponding to the trough part remains as a magnetic pattern of the ridge part.
Subsequently, as shown in FIG. 1C, carbon (C) as a first nonmagnetic material 155 is filled by sputtering to fill the trough part of the magnetic pattern. The first nonmagnetic material 155 is surface-modified and then, as shown in FIG. 1D, carbon (C) as a second nonmagnetic material 156 is film-formed by sputtering on the first non-magnetic material 155. Thereafter, ion milling is performed, whereby, as shown in FIG. 1E, the second nonmagnetic material 156 and the first nonmagnetic material 155 are etch-backed. Finally, carbon (C) is again deposited by a CVD method to form a protective layer 157 (see, FIG. 1F). When a lubricant is coated on the protective layer 157, an objective discrete track medium is obtained.
FIG. 2 is a cross-sectional view of the discrete track medium described in JPP'067. In this medium, as shown, a soft magnetic layer 202, a magnetic recording layer 203 and a protective layer 205 are sequentially stacked on a nonmagnetic substrate 201. The magnetic recording layer 203 after film formation is partially nonmagnetized by injecting an atom to have a nonmanetized layer 204 together. Here, the width W of the magnetic recording layer 203 is preferably 200 nm or less, the width L of the nonmagnetized layer 204 is preferably 100 nm or less. Specifically, for example, the nonmagnetized layer can be formed by, after the formation of the protective layer is completed, coating a resist on a surface of the protective layer to selectively mask the underlying magnetic recording layer and implanting an atomic ion only in the exposed magnetic recording layer by using an ion beam or the like.
FIGS. 3A to 3E are cross-sectional views sequentially showing the method of producing a bit patterned medium by etching described in JPP'091. First, as shown in FIG. 3A, a medium obtained by sequentially stacking a ferromagnetic recording layer 352, a protective layer 353 and a resist layer 354 on a glass substrate 351, and a stamper 371 having formed thereon a trough pattern corresponding to a magnetic pattern are prepared. Then, although not shown, the trough part of the stamper is pressed against the resist layer to transfer the trough-ridge pattern and the resist residue remaining at the bottom of the trough part of the patterned resist layer is removed by etching. Thereafter, as shown in FIG. 3B, ion milling is performed using the remaining resist layer as a mask, whereby the ferromagnetic layer 352 is etched.
Subsequently, as shown in FIG. 3C, carbon (C) as a first nonmagnetic material 355 is formed as a film by sputtering to fill the trough part of the magnetic pattern. After filling of the trough part is completed, as shown in FIG. 3D, the nonmagnetic material 355 is ion-milled to etch-back the nonmagnetic material 355. Finally, carbon (C) is again deposited by a CVD method to form a protective layer 356 (see, FIG. 3E). When a lubricant is coated on the protective layer 356, an objective bit patterned medium is obtained.
These conventional patterned mediums have many points to be improved for more increasing the magnetic recording density. For example, in the discrete track medium, magnetic separation between adjacent tracks must be performed sharply, and in the bit patterned medium, magnetic separation between patterned magnetic bits needs to be improved.
Specifically, in the conventional technique of performing the patterning of the medium by etching as described above, because of indistinct boundary line of the track, the trough formed between adjacent tracks can be hardly formed in a sharp groove shape and there is a problem in coping with increase in the density. Also, a bump generated due to back filling of the trough prevents contact with, for example, a readout head such as GMR and TMR and therefore, the surface needs to be flattened by etch back. Furthermore, a step for etching or back filling is added to the process of forming a magnetic recording medium and this gives rise to a problem of rise in the production cost.
On the other hand, in the conventional method by ion implantation, a CoCr-based alloy is used in the magnetic recording layer and therefore, ion implantation of a large amount of a nonmagnetic element is required to weaken the magnetism of the implanted region, which causes a problem that the surface property of the medium deteriorates or the production efficiency decreases. Also, since the amount of ion implanted in the ion implantation region is fairly large, a trouble of bumping of the regional portion arises. Particularly, in the method by ion implantation, the volume of a bit is made small to cope with increase in the density, but the magnetic energy decreases in proportion and there arises a problem that the magnetism disappears due to a thermal fluctuation phenomenon. In order to solve this problem, a material having high magnetic anisotropy is used with an attempt to increase the magnetization energy, but the coercive force Hc becomes high and this brings about a new problem that writing through the head cannot be performed. A so-called thermal assist recording method (the medium is heated only at writing, whereby Hc is decreased) has been proposed to solve such a problem, but the head structure is complicated and this method is not recommendable.